Microcapsules are defined as small particles of solids, or droplets of liquids, inside a thin coating of a shell material such as beeswax, starch, gelatine or polyacrylic acid. They are used, for example, to prepare liquids as free-flowing powders or compressed solids, to separate reactive materials, to reduce toxicity, to protect against oxidation and/or to control the rate of release of a substance such as an enzyme, a flavour, a nutrient, a drug, etc.
Over the past fifty years, the prior art has concentrated on so-called “single-core” microcapsules. However, one of the problems with single-core microcapsules is their susceptibility to rupture. To increase the strength of microcapsules, it is known in the art to increase the thickness of the microcapsule wall. However, this leads to a reduction in the loading capacity of the microcapsule. Another approach has been to create so-called “multi-core” microcapsules. For example, U.S. Pat. No. 5,780,056 discloses a “multi-core” microcapsule having gelatine as a shell material. These microcapsules are formed by spray cooling an aqueous emulsion of oil or carotenoid particles such that the gelatine hardens around “cores” of the oil or carotenoid particles. Yoshida et al. (Chemical Abstract 1990:140.735 or Japanese patent publication JP 01-148338 published Jun. 9, 1989) discloses a complex coacervation process for the manufacture of microcapsules in which an emulsion of gelatine and paraffin wax is added to an arabic rubber solution and then mixed with a surfactant to form “multi-core” microcapsules. Ijichi et al. (J. Chem. Eng. Jpn. (1997) 30(5):793-798) microencapsulated large droplets of biphenyl using a complex coacervation process to form multi-layered microcapsules. U.S. Pat. Nos. 4,219,439 and 4,222,891 disclose “multi-nucleus”, oil-containing microcapsules having an average diameter of 3-20 μm with an oil droplet size of 1-10 μm for use in pressure-sensitive copying papers and heat sensitive recording papers. While some improvement in the strength of microcapsules may be realized by using methods such as these, there remains a need for microcapsules having good rupture strength and good oxidative barrier to the encapsulated substance, preferably in conjunction with high load volumes. Illustrative of this need is the current lack of commercially available ‘multicore’ microcapsules.